The science magazine of Imperial College

Speedy science

1. How do spiders walk upside-down?

Spiders attach themselves to surfaces via the tiny, flexible hairs at the end of their legs. They are estimated to have around 600,000 of these hairs on their legs at any one time. Even the smoothest of surfaces to the human eye will have cracks and crevices for these spider hairs to adhere to. Additionally, a 2011 PLoS study suggests spiders secrete an adhesive substance to allow them to attach themselves to any surface – which means, unfortunately, they can also hang out on the ceiling above your bed.

2. How do barcode scanners avoid mistakes?

The barcodes we see on products on the shelves are made up of either a 12-digit ‘Universal Product Code’ or a 13-digit ‘European Article Number’, and the pattern of lines of varying widths above them. The code can be divided up into the manufacturer number and item number. The last digit of both types of these codes is a ‘check number’ to ensure the code has been scanned correctly. Barcode scanners perform a calculation of the numbers in the UPC code, and will only scan if this calculation yields a multiple of ten.

3. Why do old books smell different to new books?

The smell of old books is one of the most recognisable and evocative smells, and is due to various volatile organic compounds (VOCs) contained in the pages, ink and adhesive. These VOCs break down over time, releasing their distinctive odours. Certain chemicals are related to certain scents, for example benzaldehyde produces a slight almond smell, and vanillin (unsurprisingly) gives off a vanilla scent. Other odours that the book has been exposed to (like smoke) are also incorporated into its smell. New books also contain these VOCs, but they have not broken down so much, and different materials, with different VOCs, may have been used in modern books.

4. Why do chillis taste hot?

Chillis taste hot because they contain a chemical called capsaicin. Capsaicin activates an ion channel receptor called TrpV1, a type of vanilloid receptor. TrpV1 is also activated by heat. When the receptor is activated, it doesn’t ‘know’ whether this is because of a chemical or because of heat, the signal is the same for both, and a capsaicin-containing chilli gets signalled as ‘hot’. Mint tastes cold for a very similar reason: it contains menthol which triggers a receptor normally activated by cold temperatures.

5. Why do leaves change colour in autumn?

As the days get shorter in the autumn, we see our deciduous trees turn from green to gold to red, before shedding their leaves altogether. Leaves are green because of the abundance of the pigment chlorophyll, which is important in photosynthesis. With less sunlight, there is less production of chlorophyll and it begins to break down. Slowly, other pigments, such as yellow carotenoids, can be seen – no longer masked by chlorophyll’s green. As the sugars stored in the leaves break down, leaves can often turn red due to the pigment anthocynanin. Temperature can also affect leaf colour in autumn: the most vivid colours are yielded from sunny days followed by cold nights.

6. Why is the sound of nails on a chalkboard so horrible?

Hearing someone scrape their nails on a chalkboard makes most people squirm and some physically hurt. There are several theories of why this particular sound is so horrible. One is that the frequencies of this sound (2-4kHz) are similar to lots of acoustic features of human speech, which the ear has evolved to amplify. Therefore, it’s simply that the ear amplifies these frequencies to a level that becomes painful. Another theory is that the particular frequencies that we find so painful are similar to primate warning calls, and hearing these frequencies gives us an automatic negative warning feeling. But these explanations are not necessarily mutually exclusive: these particular frequencies could have originally evolved to be amplified by the ear because they are warning calls, making an already aversive sound even more painful by amplifying it.

Illustrations by Kate Whittington

Iona Twaddell and Jennifer Toes are studying for an MSc in Science Communication; Kate Whittington is studying for an MSc in Science Media Production